Preheat oven to 325 degrees. Prepare a 9X13 pan with nonstick spray.
In a large bowl, combine the cake mix, oil, eggs, buttermilk, and butter.
15 oz yellow cake mix, 3/4 cup buttermilk, 4 medium sized eggs, 3/4 cup vegetable oil, 1/3 cup butter
After the batter is well combined, pour half of it into your prepared pan.
In another bowl, combine the brown sugar, nutmeg, and cinnamon.
1 cup brown sugar, 1 tbsp ground cinnamon, 1/4 tsp ground nutmeg
Sprinkle the brown sugar mixture on top of the batter in your pan, covering the entire area. Top with the remaining cake batter.
Using a knife, swirl the top layer of batter and brown sugar mixture together to make a pretty swirl design
Bake for 40-50 minutes.
While baking, prepare the icing by mixing the powdered sugar, milk and vanilla together.
2 cup powdered sugar, 4 tbsp milk, 1 tsp vanilla extract
When the cake comes out of the oven, pour the icing all over the warm cake.
Cool for 15-20 minutes before cutting.
Serve & enjoy!